In drilling wells for oil and gas exploration, understanding the structure and properties of the geological formation, including its density, provides information to aid such exploration. Logging is the process used most often to make measurements (via sensors located down hole) of formation characteristics to provide this information.
Some of the sensors used in logging operations include sonic and ultrasonic sensors. While attempts have been made to determine formation density from sonic measurements, the results have been unreliable, because borehole irregularities along the length of the transmitter-receiver array can greatly affect the final result.